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Show POSITION OF THE POBPHYPIES IN CLASSIFICATION. 97 Texture, then, if the foregoing views be true, is associated with the genesis of rocks and is determined by the conditions under which the rocks have solidified. Although it may seem to* be a trivial character, in reality it is a very important one, since it is an index of conditions and occurrences of vital importance to the genesis of the rocks and their geological relations. For it is of the highest geological importance to know whether certain rocks have been erupted or have been formed in situ; whether they are indigenous or exotic. The indications given by texture may be uncertain at times, and occasionally even misleading; but on the whole, so far as they are now understood, they may be relied upon. The differences of texture have heretofore been employed chiefly to distinguish the eruptive from the non-eruptive igneous rocks. The wholly crystalline are non-eruptive ; the partially crystalline are eruptive. But, although the wholly crystalline rocks are not commonly found in the form of lava sheets or coulees, they are occasionally found in the form of intrusions, and so, also, are the partially crystalline rocks. The intrusive condition is, therefore, a kind of intermediate stage between the eruptive and non-eruptive condition, representing an abortive attempt at eruption, sometimes resulting in a slight displacement of the magma, sometimes almost accomplishing an outpour. In very many casesâ€"probably in many more than we are now justified in affirmingâ€"this qualified eruption is associated with a texture which seems to be characteristic of it, the porphyritic texture. A satisfactory definition of "porphyry" is almost impossible to find. The most general conception is that it applies to a rock consisting of crystals, usually feldspar and quartz, imbedded in an "unindividualized" paste or base; but forty-nine-fiftieths of all intrusive and eruptive rocks come fully within such a definition. Except an insignificant quantity of obsidians and aphanitic rocks, all volcanics are decidedly porphyritic. And yet lithologists employ the term to designate a group of rocks different from volcanics, not only in their geological relations, but in their appearance as dependent upon texture. There are certainly some rocks which we do not hesitate to call porphyry, and regard them as being quite distinct from the common lavas; the distinction, moreover, being a textural and not a chemical one. As nearly as we can reach a description of the spe-7 h p |